docs: def respond(self, response_adu):
          """ Send response ADU back to client.
  
          :param response_adu: A bytearray containing the response of an ADU.
          """
          log.info('--> {0} - {1}.'.format(self.client_address[0],
                   hexlify(response_adu)))
          self.request.sendall(response_adu)